A critical determinant of interindividual differences in HIV-1 susceptibility and disease progression rates to AIDS is CCR5 expression levels. Delineating the mechanisms that modulate intersubject differences in CCR5 expression levels is thus of indisputable importance and significance for understanding HIV-AIDS pathogenesis, development of effective strategies for treatment, epidemiological studies and vaccination trials. During the past decade, studies from several laboratories including ours have underscored the critical role of genetic variation in the CCR5 locus in variable HIV-AIDS susceptibility. In the past funding cycle, our studies have unraveled both genetic and novel epigenetic mechanisms that influence CCR5 expression. For example, our studies reveal that epigenetic mechanisms such as DNA methylation of specific regulatory regions of CCR5 are key correlates of T cell-type specific and inter-individual differences in CCR5 expression. To extend these studies we propose 3 aims. Specific Aim (SA) 1a will test the hypothesis that methylation status of CCR5 promoter 2 and intron 2 is a determinant of inter-individual differences in CCR5 surface expression levels, independent of CCR5 genotype and other relevant parameters known to influence CCR5 expression (e.g. T cell activation/CCL3L dose). SA1b will test the hypothesis that epigenetic reprogramming/modification of CCR5 DNA methylation status results in repression of CCR5 gene and surface expression. SA #2is a bench to bedside proof-of-principle translational aim that will test the hypothesis that CCR5 DNA methylation status is associated with relevant HIV-AIDS endpoints in well characterized cohorts. SA #3 will test the hypothesis that there are inter-subject differences in the permissiveness of regulatory regions of CCR5 to undergo epigenetic modifications after T cell activation. The proposed research is significant because it will provide critical insights into the complex interplay between the epigenetic and genetic factors that determine variability in CCR5 expression levels, a key phenotype that influences HIV-AIDS pathogenesis. These studies have translational (bench to bedside) utility as they may identify epigenetic correlates of CCR5 that may impact on variable HIV-AIDS susceptibility. These epigenetic signatures of CCR5 expression may also have utility for targeting CCR5 expression, i.e., by epigenetic programming of key methylation sites that influence CCR5 expression.